PARENT SESSION
Ovulation

THE GONADOTROPIN SURGE UPREGULATES THE EXPRESSION OF PROGESTERONE RECEPTOR IN BOVINE PERIOVULATORY FOLLICLES.

Jo, M¹, Komar, C¹, Fortune, J¹, ¹

ABSTRACT- In cattle, intrafollicular progesterone increases rapidly and transiently within a few hours after the LH surge; a second increase occurs near the time of ovulation. In mice deficient in progesterone receptor (PR), ovulatory size follicles develop but fail to ovulate, providing evidence for an essential role for progesterone and PR in ovulation in mice. However, nothing is known about the expression of PR or its mRNA in bovine preovulatory follicles. The objective of the first experiment was to determine if and when PR mRNA and protein are expressed in bovine follicular cells during the periovulatory period. On Day 6 or 7 of the estrous cycle, heifers were injected with PGF2 to induce luteal regression and a follicular phase and 36 h later, a GnRH analogue was administered to induce the LH/FSH surge. Follicles were collected at 0, 3.5, 6, 12, 18, and 24 h after GnRH injection (n = 3/time). Two pieces of follicle wall (theca + attached granulosa cells) were frozen for immunohistochemistry and total RNA was extracted from the remainder. RNase protection assays revealed a transient increase in the concentration of PR mRNA that peaked at 6 h after GnRH injection (15-fold; P < 0.01), a decline to time 0 levels by 12 h, and a second increase by 24 h (6-fold; P < 0.05). PR was immunolocalized to the nuclei of both theca and granulosa cells of periovulatory follicles. The objective of the second experiment was to determine effects of gonadotropins on PR mRNA expression in cultured granulosa cells. Granulosa cells isolated from preovulatory follicles 36 h after PGF2 injection were cultured in control medium or with a luteinizing dose of LH (100 ng/ml) or FSH (100 ng/ml) for 0, 4, 10, or 24 h (n = 4 cultures/time). LH and FSH increased PR mRNA expression in a time-dependent manner (P < 0.01); the highest expression was observed at 10 h (2- to 2.5-fold higher than control cultures) and expression had declined by 24 h of culture. These results show that the gonadotropin surge induces a rapid and transient increase in expression of PR mRNA in bovine periovulatory follicles, concomitant with the early, transient increase in follicular production of progesterone, and that this pattern can be mimicked in vitro by exposing granulosa cells to a luteinizing dose of LH or FSH. These results suggest an important role for PR in the regulation of periovulatory events in cattle.

KEY WORDS: ovulation, hormone receptors, gene expression during follicular development, gonadal function